1. Field of the Invention
The present invention relates to a turbo fan engine nacelle exhaust system having a concave primary nozzle plug for reduced shock associated noise.
2. Background Information
Referring to FIGS. 1, 2 and 3, a prior art turbo fan engine nacelle 10 is shown. In FIG. 1, the prior art turbo fan engine nacelle 10 is shown attached to a wing 12 of an aircraft. As shown in FIG. 2, the turbo fan engine nacelle 10 houses an engine 11 including a fan 14 which draws air through inlet 15, and forces the air to be exhausted through a primary nozzle 16 and fan nozzle 18. The primary nozzle 16 is bordered interiorly by the primary nozzle plug 20, and bordered exteriorly by the primary nozzle sleeve 23. The fan nozzle 18 is bordered interiorly by a core cowl 22, and bordered exteriorly by a fan nozzle sleeve 24. Accordingly, the engine 11 forces exhaust through the primary nozzle 16 and fan nozzle 18, and as shown in FIG. 1, the imperfectly expanding supersonic flow 26 exhausted from the turbo fan engine 11 interacts with turbulent air structures from the jet and creates a dominant "shock cell" noise producing region 28 which radiates sound waves 29 to the cabin, thereby producing undesirable aircraft cabin noise.
Referring to FIG. 3, the shock cell structure in the exhaust plume of an engine is the result of two flow features: (1) a compression of the secondary (fan) flow 30 on the aft section of core cowl 22 and the resulting expansion off the secondary-ambient flow shear layer 34; and (2) compression waves necessary to turn the radially contracting secondary flow 30 and primary flow 32 in the axial direction. The compression wave near the core cowl trailing edge reflects alternately off the secondary and ambient flow shear layer 34 as an expansion wave and the primary-secondary shear layer 36 as a compression wave. This initial wave reflection pattern is reinforced in the flow turning region 38 above and immediately aft of the primary nozzle plug 20, thereby resulting in higher (shock cell) strength for the compression and expansion waves between the shear layers 34,36.
The common solution to the cabin noise problem is to add insulation to the aircraft. However, additional insulation adds significant weight which reduces fuel economy of the aircraft and also reduces passenger carrying capacity of the aircraft.
Accordingly, it is desirable to provide a turbo fan engine nacelle exhaust system which produces lower shock cell noise to the cabin without requiring additional sound insulation in the aircraft.